In various types of optical systems, the constituent optical elements such as lenses, filters, and/or mirrors are impinged with the radiation with which the system is used. If an optical element absorbs some of the incident radiation and especially if the incident radiation is intense, the element likely will experience a significant increase in temperature. Such a temperature change can thermally distort an optical element, for example the reflective surface of a mirror. With many types of optical systems, the intensity of radiation is normally too low to cause significant heating of the elements, the system can continue to function satisfactorily despite being heated, or any thermal-distortion effects of heating can be accommodated without any significant degradation of system performance. But, in other optical systems, especially systems used for extremely demanding imaging applications and the like, thermal distortion of one or more optical elements can degrade the system's overall optical performance to below specifications.
Certain types of optical systems are designed and constructed to such extremely tight dimensional and geometrical tolerances that serious attention must be directed to avoiding excessive heating of the constituent optical elements. Examples of such systems are astronomical telescopes, certain types of space-borne optical systems, high-power laser systems, and microlithography systems. Indeed, many types of optical systems that normally operate in a vacuum probably could benefit from such attention.
Most current microlithography systems use wavelengths of deep ultraviolet (DUV) light (λ=150 to 250 nm) for imaging purposes. To achieve further improvement of imaging resolution, substantial research is being directed to the development of practical microlithography systems that use “extreme ultraviolet” (EUV) wavelengths, in the range of 11 to 14 nm. Whereas optical systems (such as projection-optical systems) for use with DUV light are usually mostly to fully refractive, no materials are currently known that are sufficiently transmissive to EUV light and that exhibit a usable refractive index to EUV light for use in making EUV lenses. Consequently, current EUV optical systems are entirely reflective and usually comprise multiple mirrors each having a multilayer EUV-reflective coating on its reflective surface to provide the mirror with a usable reflectivity (approximately 70%, maximum) to EUV light at non-grazing angles of incidence.
Most practical EUV sources are very intense and radiate a large amount of energy. (Also, a large effort is currently underway to increase the intensity of EUV sources, particularly portable EUV sources, substantially over current sources.) EUV-reflective mirrors often experience heating during use because their multilayer reflective coatings absorb a substantial amount (with current mirrors, approximately 30% or more) of the incident EUV radiation. The mirror situated closest to the EUV source, such as the most upstream mirror in an illumination-optical system, typically absorbs more energy than any other mirror of the system. By way of example, in EUV microlithography systems currently under development for high-throughput use, the radiant-energy load for the mirror closest to the EUV source can be 1 kW or greater. This radiant load typically includes EUV light in the desired wavelength band as well as substantial out-of-band (OoB) light. Similarly, mirrors used in other high-power optical systems, such as certain laser systems, experience substantial heat loads. As the mirror absorbs energy from incident light, the mirror temperature increases. If precautions are not taken under such conditions, the mirror can experience thermal effects (e.g., expansion) that can cause an unacceptable degradation of optical performance of and possible fracture or other damage to the mirror.
To reduce thermal effects on mirrors in EUV systems, at least some of the mirrors are conventionally made of a material having a very low coefficient of thermal expansion (CTE). An exemplary low-CTE material used for making conventional EUV mirrors is ZERODUR®, made by Schott, Germany. Unfortunately, this and other low-CTE materials tend to have low thermal conductivity, which poses a challenge in removing heat at a desired rate from the reflective surface of the mirror.
A mirror of which the body is made substantially of a single material typically has a substantially uniform CTE throughout the body. If such a mirror simply experiences an overall increase in temperature, the temperature increase will be accompanied by a substantially uniform expansion of the mirror, which inevitably changes the curvature of the reflective surface of the mirror. For very demanding applications, this change can be significant. An example is shown in FIGS. 1(A)-1(B). Turning first to FIG. 1(A), the elevational profile of a conventional mirror 10 at ambient temperature is shown. The mirror 10 has a body 12, a base 14, and a reflective surface 16. The reflective surface 16 is concave at a particular radius of curvature. Heating the mirror 10 causes the mirror to expand, which results in a slight change in the radius of curvature of the reflective surface 16. The result of this change is shown in FIG. 1(B), showing the ambient-temperature mirror 10 (at the same scale as depicted in FIG. 1(A)) and a “warmer” mirror 20 (shown larger to highlight the effect). The reflective surface 26 of the warmer mirror 20 has an altered radius of curvature, as evidenced by the lack of superimposability of the respective profiles of the reflective surfaces 16, 26. Although the change in curvature radius may appear minor, such a change can substantially degrade the optical performance of the mirror especially if the mirror is used in an extremely high-precision optical system such as a microlithography system.
To reduce these and other thermal effects, there are several conventional approaches to removing heat from the mirror during use. One conventional method involves simply allowing the heat to radiate from the mirror. This method is inefficient and can provide an inadequate rate of cooling, especially of a mirror located close to the source of radiant energy. Another method involves mounting the mirror to a mass to which heat is conducted from the mirror, such as via the mirror mountings, for example. This method is also inefficient for high heat loads and can subject the mirror to high thermal and/or mechanical stresses.
Yet another conventional approach involves cooling the mirror with a temperature-regulated liquid circulated through cooling channels defined in the mirror body. This approach as currently implemented has several problems. First, it is difficult to form the channels in the mirror body, especially without having to fabricate the body of multiple pieces that are bonded together. Second, cooling channels inevitably form different thermal gradients in different portions of the mirror, such as one thermal gradient in the upper portion between the irradiated reflective surface and the cooling channels, and another thermal gradient in the lower portion between the cooling channels and the base of the mirror. Consequently, despite the mirror being liquid-cooled, the upper portion still exhibits greater thermal expansion than the lower portion, which changes the curvature of the reflective surface. Third, to prevent undesirable changes to the reflective surface (e.g., “print-through” of the cooling channels to the reflective surface as the reflective surface is being machined), the cooling channels must be located some distance, in the thickness dimension of the mirror, from the reflective surface. Since the reflective surface is where the cooling channels are most needed, any significant thickness of mirror body between the cooling channels and the reflective surface produces thermal gradients. Fourth, especially if the reflective surface has curvature, it is extremely difficult or impossible using cooling channels to achieve a uniform rate of heat removal from all portions of the reflective surface, simply because the body thickness between the curved reflective surface and the cooling channels is not uniform. Hence, different thermal gradients are established across the mirror that typically produce greater thermal expansion of hotter portions of the mirror (e.g., between the reflective surface and the cooling channels) relative to cooler portions. These differential expansions can produce substantial stress in the mirror and can cause unacceptable changes in the curvature of the reflective surface.
Yet another challenge to liquid cooling a mirror is the manner in which the liquid is circulated through the channels. More specifically, whereas turbulent flow of the liquid through the channels can provide for efficient heat transfer and cooling than laminar flow, turbulent flow often generates vibrations within the mirror. These vibrations may be transmitted through the microlithography system, which can compromise the accuracy of microlithographic processes performed by the system.
Increasing the flow rate of the coolant through the mirror body can reduce the rate of temperature rise and the overall temperature rise of the mirror. But, increasing the flow rate may generate turbulence, and increasing the flow rate also usually does not yield any substantial change in the temperature gradients between the reflective surface and the coolant channels.
One conventional approach to reducing temperature gradients is making the mirror of a material having high thermal conductivity. However, the available materials satisfying this criterion tend to have larger CTEs, wherein a combination of high thermal conductivity and high CTE tends to produce relatively large temperature rises of the mirror during use, and consequent significant changes in mirror shape. Another conventional approach is to make the mirror of a material having a low CTE to reduce the overall expansion of the mirror during heating. However, the few available materials satisfying this criterion tend to have lower thermal conductivity. Consequently, heating the reflective surface of the mirror tends to increase the temperature gradients in the mirror (reflective surface versus the mirror body).
Yet another conventional approach to mirror cooling involves mounting the mirror's rear surface to a cooling plate. The cooling plate is actively cooled by circulating temperature-controlled liquid through cooling channels or passages formed in the plate. Unfortunately, even with such a cooling plate, the reflective surface of the mirror changes shape whenever a heat load is applied to it, because: (a) there remains a temperature gradient between the reflective surface and the cooling plate that causes the mirror to bend and, with a concave reflective surface, increase its radius of curvature; and (b) the entire mirror heats up and expands, which increases the radius of curvature of a concave reflective surface. Again, low-CTE materials reduce this problem, but they have disadvantages as discussed above.
Excessive heating of the reflective surface of a mirror also can damage the coating(s) on the surface. Furthermore, heating the reflective surface can increase radiative heat transfer from the mirror to other surfaces and components in the optical system, which can have a degradative effect overall.
Therefore, a need exists for mirrors and other optical elements, used in high-intensity optical systems and other systems in which the elements may undergo substantial heating, that exhibit reduced changes in their optical surfaces (and thus in their optical performances) while withstanding their conditions of use.